1. Field of the Invention
This invention resides in the field of membranes for reverse osmosis, with particular interest in the desalination of water. This invention also relates to the uses of nanotubes and to structures incorporating nanotubes.
2. Description of the Prior Art
Porous polymeric membranes have been widely studied as reverse osmosis media for the purification of fluids. The most common reverse osmosis membranes are those utilizing a solution-diffusion mechanism in which the species of interest (water in the case of desalination) both dissolves in and diffuses through the membrane material at a faster rate than other species in the feed fluid (for example, salt). Examples of such membranes are those disclosed by Cadofte, J. E., U.S. Pat. No. 4,277,344, issued Jul. 7, 1981, and Hoek, E., et al (The Regents of the University of California), International (PCT) Patent Application Publication No. WO 2006/098872 A2, publication date Sep. 21, 2006. Unfortunately, membranes operating by the solution-diffusion process provide only a low flux of the desired species, and although the flux can be increased by modifications of the membrane parameters, such modifications are typically detrimental to the rejection performance of the membrane, i.e., they tend to lower the purity of the permeate.
The use of nanotubes presents an alternative to the traditional solution-diffusion polymeric membrane since nanotube channels themselves offer selectivity in molecular transport. Water molecules, for example, pass through nanotube channels at a significantly faster rate than salt ions or other species whose molecular size exceeds that of water. Prominent among the nanotube-containing structures that are designed for this purpose are those in which the nanotubes are attached to a silicon chip, as disclosed by Holt, J., et al (Regents of the University of California), International (PCT) Patent Application Publication No. WO 2007/025104 A2, publication date Mar. 1, 2007, and by Holt, J. K., et al., “Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes,” Science 312, 1034-1037 (19 May 2006). The membranes in these disclosures are formed growing a dense, vertically-aligned array of double-wall carbon nanotubes (DWCNTs) on the surface of the chip by chemical vapor deposition (CVD), followed by the deposition of silicon nitride, also by CVD, to fill the gaps between the DWCNTs.